1. Field of the Invention
The present invention relates to a drill screw for producing a threaded hole and allowing itself to anchor therein. More particularly, the present invention relates to a drill screw of such nature whose drill head or bit is shaped so as to allow the same to be cold forged.
2. Description of the Prior Art
In general, drill screws can be classified roughly into two types; the first type has a drill bit shaped by a cutting machine such as a milling machine, and the second type has a drill bit shaped under pinch pointing or cold forging. With increasing demands for drill screws, the second type tends to outnumber the first type because of its superior productivity and economy.
To increase the drilling capability most of the forged screws is shaped so as to have relief and twist in its cutting surface as is in conventional all-purpose drills. Examples of this type are disclosed in U.S. Pat. No. 3,079,831 and U.S. Pat. No. 3,241,426 (Japanese Patent Publication No. 45-24728). However, difficulties arise as to how to fabricate a molding die adapted to shape such configuration under cold forging. Even if it is well fabricated, the die cannot withstand a long use. A frequent replacement will be needed.
To overcome the difficulties pointed out above, there is a proposal for producing a flat cutting surface free from any relief and twist, wherein the cutting surface extends axially of the drill. This facilitates to fabricate a die and prolongs its life. Typical examples of this type are disclosed in U.S. Pat. No. 3,463,045 (Japanese Patent Publication No. 47-2562), U.S. Pat. No. 3,395,603 (Japanese Patent Publication No. 48-13139) and U.S. Pat. No. 3,710,676.
However, this proposal only aims at securing the easiness of fabricating a die, and fails to consider the function of a drill screw. As a result, the drill screws shaped in such dies must have a reduced drill capability.
The present invention is directed toward solving the problems pointed out with respect to the known drill screw and has for its object to provide an improved drill screw whose drill bit is uniquely shaped so as to allow the same to be cold forged without trading off its drilling function.
Other objects and advantages will become apparent from the following description given hereunder, it should be understood, however, that the detailed description and specific embodiments are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description.